Detrital zircon U-Pb dating of Majiajie Group and its tectonic implications

Majiajie Group,exposed in the southwestern part of the Huanan Uplift in the Jiamusi Massif,in eastern Heilongjiang Province,northeastern China is a suit of contact metamorphic rocks,which is characteristics of metamorphic mineral zonation from staurolite,andalusite,garnet to biotite from the west to the east. The LA-ICP-MS zircon U-Pb dating of the andalusite-bearing carbon slate and garnet quatz schists from Majiajie Group give the three age populations: 272 ~ 310 Ma,479 ~ 533 and 800 Ma. The two rock samples with the minimal age population of 272 ~ 310 Ma show a similar peak age of 276 Ma and 279 Ma and the youngest age of 272 Ma,suggesting that the deposition time of the Majiajie Group was before the Middle Permian. The zircon age of the granite intruding the Majiajie Group is 259 Ma,suggesting that the time of contact metamorphism of the Majiajie Group occurred in the early Late Permian.The age population of 479 ~ 533 Ma in the two samples gives a similar peak ages of 499 Ma and 522 Ma,which are the most important age of high-grade metamorphism and granitic intrusion in the Jiamusi Massif. The age population of 800 Ma shows several peak ages of 975 Ma,1063Ma,1284 Ma and 1422 Ma,which suggest that there were Precambrian crust in the source region for the Majiajie Group. These evidences indicate that the Majiajie Group was a suit of contact metamorphic rocks related to the early Late Permian granitic intrusion,instead of the Precambrian metamorphic rock series. In view of the facts that the zircon ages of 479 ~ 533 Ma and 800 Ma are widespread in the Mashan Group and the Early Permian volcanics with the zircon ages of 276 ~ 279 Ma are widely distributed in the eastern margin of the Jiamusi Massif,it is concluded that the Mashan Group as the basement and material source made great contributions to the formation of the Majiajie Group,and the Early Permian volcanic rocks made a certain contribution to the deposition also.